It is common in foundry operations to divide the smelting of metals into two operations, melting metal and holding melted metal in the molten state. Induction furnaces can be used for both operations.
To begin, a metal charge is melted in an induction furnace by applying electromagnetic power to the induction coil of the induction furnace. Melting requires a high power density (500 to 1000 kW per ton of metal) and, typically, the melting operation lasts from 25 to 40 minutes.
Following melting, holding the molten metal for treatment is often required prior to casting or pouring. During treatment, the necessary samples are taken and tested, alloy elements are added, and sometimes unwanted elements are removed from the molten metal. Sometimes the furnace is discharged very slowly during pouring to facilitate the requirements of the pouring process. During treatment and pouring, the required temperature for the molten metal must be maintained, and this is usually done by applying limited power to the induction coil of the induction furnace.
Clearly, melting and then holding the molten metal in the same furnace is an inefficient use of equipment. Thus, to better utilize equipment, melting and holding operations are usually carried out simultaneously in two adjacent furnaces. This can be done in several ways.
The first way is to use one induction coil power supply connected through a system of switches to two furnaces. To melt the charge in one furnace, the power supply is connected to the induction coil for that furnace. When melting is complete, the power supply is switched over to the other furnace, which has been loaded with cold charge, with no further power being supplied to the first furnace. Treatment and pouring of the molten metal in the first furnace is carried out without power. To prevent the molten metal from becoming to cool, it must be superheated when melting to account for temperature losses during holding. This requires a great deal of power to be applied during melting, and leads to inaccuracies in the final metal temperature after treatment.
A second approach is to use two furnaces each connected to its own dedicated power supply. This allows more flexibility than the first method since, when melting is complete, the power applied to the furnace can be reduced considerably to a level sufficient to hold the molten metal at the desired temperature. This also allows more accuracy, since the temperature of the molten metal can be controlled by the application of power to the furnace during holding. A disadvantage of this method is that the equipment is under-utilized. Since each power supply is capable of supplying full melting power, which greatly exceeds holding power, the result is that over 50% of the time the power supply is utilized to only about 10% of its capacity.
A third technique, which is described in U.S. Pat. No. 4,695,316, combines the above two approaches. In the third technique, one power supply is connected to two furnaces through a switching network. While one furnace is energized for melting, the other holds a previously melted charge. During the holding process, the switches are actuated to apply power to the holding furnace for a short time to maintain the temperature of the molten metal. Thereafter, the switches are actuated to apply power to the melting furnace to continue melting. Although this method provides temperature maintenance during holding and better utilization of equipment, it does have some drawbacks. The switches need to be operated extensively and are subject to premature wear. In addition, the metal in the holding furnace sometimes becomes superheated, with undesired effects.
It is an object of the invention to provide a separation of function between melting and holding furnace to increase equipment utilization, but without the disadvantages of prior techniques.